Ash in the form of coal fly ash and of municipal solid waste (MSW) fly ash is a significant environmental problem. It is estimated that power plants in the United States generate 80,000,000 tons of fly ash by burning coal. Only about 20% of this amount is utilized and the rest is landfilled. Coal fly ash has complex chemical composition containing up to 80 weight percent of SiO.sub.2 and Al.sub.2 O.sub.3, with Fe.sub.2 O.sub.3, CaO, MgO, Na.sub.2 O, K.sub.2 O, SO.sub.3 and other oxides as the remainder. It may also contain traces of hazardous elements such as arsenic, lead, barium, and mercury. The chemical composition significantly varies depending on coal deposits and power plant operating parameters. In addition to coal fly ash, large quantities of municipal solid waste (MSW) ash are being produced by the incineration of municipal trash, garbage, and even sewage solids.
Fly ash has been used in cement as a substitute for shale; in concrete as a substitute for cement and sand, and as aggregates; in road construction as a filler to bitumen, and as a substitute for sand in the foundation layer; in bricks as a substitute for clay; for soil stabilization; etc. These prior art uses do not lead to massive consumption of fly ash because the ash is usually only a minor component in these composites. It would be desirable to provide processes and products that would use fly ash as the major component, preferably in amounts exceeding 85 weight percent.
Additionally, it is important that the fly ash products be cost competitive to produce. This is particularly true in the case of bricks or aggregate materials where the price competition is fierce. It is therefore desirable that the processing occur a lower temperatures than are used in conventional manufacturing. The lower temperatures save money by using less energy and by requiring less expensive equipment. Cost should also be kept down by using low cost materials as additives.
Finally, the fly ash products should be of good quality as judged by appearance and color and physical properties such as compressive strength, bending strength, impact strength, modulus of elasticity, abrasion resistance, thermal conductivity (R-value), thermal expansion coefficient, water stability/leachability, freeze/thaw resistance, etc.